John S. Plaskett's father died when he was 16, and he quit high school to help tend the farm. He later worked as a mechanic, and was eventually hired as a mechanic and handy-man by the physics department at the University of Toronto. There his curiosity was ignited, and at the age of 30 he enrolled as a student, graduating in 1899. He was hired at Ottawa's Dominion Observatory in 1903 -- still as a mechanic -- but in 1905 he was promoted to astronomer, beginning his career at the age of 40.

In 1922 he discovered Plaskett's star (or Plaskett's twins), a massive binary star previously thought to be a single star. He confirmed Jan Hendrik Oort's model of galactic rotation, added evidence to support Arthur Eddington's theory that interstellar matter is widely distributed throughout the Galaxy, and showed that the stationary lines of ionized calcium in the spectra of hot stars are caused by clouds of interstellar gas. He lobbied for construction of the much larger Dominion Astrophysical Observatory in Victoria, and designed its 72-inch telescope, at the time the largest telescope in the world. He also conducted important work on radial velocities, the spectra of O and B-type stars, and spectroscopic binaries.